The fluid catalytic cracking (FCC) process has achieved widespread utility in the petroleum refining industry for producing low boiling point hydrocarbon products, especially gasoline from relatively higher boiling feeds. The FCC process has, in fact, achieved a preeminent position in the refining industry in the United States for catalytic this purpose and now accounts for almost all the non-hydrogenative, cracking capacity in the industry. In current cracking units, the cracking feed, generally a gas oil from the vacuum distillation tower, is brought into contact with a hot cracking catalyst at the foot of a tall, columnar riser in which the cracking takes place. The cracking feed travels up the riser concurrently with the catalyst and at the top of the riser, the cracking products are separated from the catalyst in the disengaging vessel, commonly referred to as "reactor", conventionally employing cyclone separators for this purpose. The cracking products are then passed to the product recovery section of the unit for separation into the various cracked fractions. The separated catalyst is passed to a regenerator in which the coke laid down by the cracking process is oxidatively removed, thus restoring activity to the catalyst and, at the same time, providing heat for the endothermic cracking process by the combustion of the coke. The regenerated catalyst is then returned to the foot of the cracking riser for contact with the cracking feed.
To optimize the cracking process it is necessary to contact the feed as uniformly as possible with the catalyst so as to procure the catalyst/oil ratio which is most favorable to the desired product yield and distribution. In practice, this requirement has given rise to a considerable number of problems arising not only from the basic difficulties of achieving uniform contact between a finely divided solid (the catalyst) and a liquid (the cracking feed) but also because cracking units are usually required to handle extremely large equantities of both these components. For example, in a unit with a nominal capacity of about 100,000 Bbl/day, about 4,000 barrels of oil pass through the unit every hour and at a typical catayst/oil ratio of 5:1, 3,200 tons of catalyst also pass by any point in the unit every hour in addition to the oil. These large quantities are difficult to handle with the utmost precision.
Proposals have been made for heating the hydrocarbon stream prior to injection into the cracking zone in the form of a vapor but not only is this uneconomic because of the high degree of preheat required it is also undesirable because it initiates undesirable, non-selective thermal cracking before the feed contacts the catalyst. It also results in excessive coking and poor product distribution even though optimum performance would be realized with an all vapor feed since the most desirable reactions occur in the vapor phase. The conventional practice has therefore been to use a liquid feed dispersed with steam with the heat of regeneration supplying heat for vaporizing the feed and for the endothermic cracking process. Unit performance can be improved by a more uniform oil feed/catalyst distribution and by atomizing the oil into droplets more closely matching the particle size of the catalyst. Droplets of 350 microns, preferably less than 100 microns, in diameter are desirable. Accordingly, there is a significant incentive for good feed atomization and various proposals have been made to achieve this.
U.S. Pat. No. 3,654,140 (Griffel) discloses a FCC feed injector which employs a spray nozzle with a helical element which imparts a circular motion to the liquid feed stream to break it up into a hollow conical sheet which disperses droplets in the cracking zone. Atomization of the feed is promoted by means of the steam which is fed into the injector nozzle in the form of an annulus around the oil stream.
The use of steam for improving the atomization of the cracking feed is, of course, well established in the industry and is described, for example, in U.S. Pat. No. 3,071,540 (McMahon) which employs a coaxial injection nozzle feeding concurrent streams of oil, steam and catalyst into the cracking reactor.
U.S. Pat. No. 3,152,065 (Sharp) describes a feed nozzle for an FCCU which has a helical form at the outlet of the nozzle to break the hydrocarbon feed up into a cone of finely dispersed droplets. Atomization of the feed is promoted by steam injected through a central injection pipe with an orifice plate facing the end of the steam conduit.
In practice, all the conventional FCC feed injectors have disadvantages of one kind or another. It may be that the degree of atomization achieved is unsatisfactory, the pressure drop required for achieving satisfactory atomization is excessive, catalyst mixing is poor or the device may not be mechanically robust or cannot be easily maintained. Alternatively, the nozzle design may place limits on the amount of steam which can be injected concurrently with the feed and this may impose limitations on the ability to achieve a given product distribution because it has been established that increased steam/oil ratios may be desirable for improved selectivity to specific products, especially gasoline, particularly for heavier feeds or higher operating pressures. There is therefore a continuing need for improved FCCU feed injection systems.